A conventional hydraulic clutch actuating system for motor vehicles has a master cylinder mounted on a pedal bracket of the motor vehicle and a slave cylinder which is fixed in the vehicle near the transmission and which is constructed conventionally or as a central release device in coaxial arrangement with respect to a transmission shaft. The master cylinder and slave cylinder are hydraulically connected together by way of a hydraulic line. The piston of the hydraulic master cylinder, which is hydraulically connected with a compensating reservoir, is operatively connected with a clutch pedal by way of a piston rod so that the master cylinder can be actuated by pressing down the clutch pedal, which causes displacement of the piston in the master cylinder. In that case, a fluid column is pushed through the hydraulic line in the direction of the slave cylinder and hydraulically loads the slave cylinder. The slave cylinder, more precisely the piston thereof, is disposed in operative connection with a release mechanism of a friction clutch by way of a piston rod via a release lever and release bearing or, in the case of the central release device with an annular piston and a sliding sleeve, directly via the release bearing seated on the sliding sleeve.
If the slave cylinder is hydraulically loaded for disengaging the friction clutch then by the piston-actuated release mechanism a clutch pressure plate is separated from a clutch driven plate of the friction clutch, the driven plate being seated on a transmission shaft and co-operating with a flywheel carried by the crankshaft of the combustion engine. As a result, separation of the combustion engine from the transmission of the motor vehicle takes place. If, on the other hand, the clutch pedal is relieved of load so as to re-engage the friction clutch, the slave cylinder, more precisely the piston thereof, is returned to its basic or start setting as a consequence of, inter alia, the spring forces of the friction clutch, in which case the above-mentioned fluid column is displaced through the hydraulic line back again in the direction of the master cylinder.
In such a hydraulic clutch actuating system—to be regarded as a quasi-static hydraulic force transmission system in which there is no continuous flow of the hydraulic fluid—vibrations are transmitted from the combustion engine, particularly the crankshaft thereof, via the components of the friction clutch, the release bearing, in certain circumstances the release lever, and the slave cylinder to the fluid column present in the hydraulic line between the slave cylinder and master cylinder, in which the vibrations propagate as pressure pulsations.
It is already regarded as disadvantageous that these pressure pulsations can be sensed by the driver as vibrations at the clutch pedal—commonly referred to as “tingling”—particularly when in typical city driving a foot rests on the clutch pedal or the depressed clutch pedal is held, for example, during a stop at traffic lights.
There is no lack of proposals in the prior art on how to deal with this problem (for example, DE 36 31 507 C2, DE 40 03 521 C2, U.S. Pat. No. 5,816,046, 6,647,722 or 7,107,768). It is common to these proposals that a separate subassembly, which does not interrupt the fluid column between master cylinder and slave cylinder and which in general is also capable of satisfactorily damping the pressure pulsations, for vibration damping is inserted or arranged in or parallel to the hydraulic line between master cylinder and slave cylinder. However, the prior art solutions in part demand a relatively large amount of installation space, which is not always available to sufficient extent in the engine compartment of the motor vehicle, and/or require a relatively complex and accordingly expensive construction of the device, which is not desirable for mass production.
Against this background there was proposed in document U.S. 2011/303315 (FIGS. 17 and 18), U.S. Ser. No. 13/145,408 filed on Aug. 8, 2011 which forms the preamble portion of claim 1, in particular a device for reducing pressure pulsations, the device having a housing in which is formed a pressure chamber able to be disposed in fluid connection with a hydraulic actuating system by way of two connections. In this reference—as one of the proposed damping measures—the pressure chamber is bounded by a resilient membrane having a pressure-loadable surface facing the pressure chamber and a surface remote from the pressure chamber. The surface remote from the pressure chamber is in that case provided with a profiling by way of which the membrane when loaded with pressure can be supported at a fixed wall section of the housing and which has a plurality of web sections each with a plurality of webs. Each web has a web foot distant from the wall section of the housing, a web end near the wall section and a defined web cross-section. This profiling shall, under pressure-loading of the membrane and by pressing against the fixed wall section of the housing, produce—in dependence on the static prevailing pressure and the amplitude of the pressure pulsation—a defined, limited volume take-up in the pressure chamber, which in turn influences the damping characteristics of the device.
In general, it can be said with respect thereto that the higher the possible volume absorption by the volume absorbing component—here the resilient membrane with the profiling thereof, i.e. its membrane contour—the more effectively a pressure pulsation in the fluid column is damped. However, on the other hand the overall volume absorption of the hydraulic actuating system has to be kept as small as possible so as to not make the actuating system too “soft” and to avoid, due to pedal travel losses accompanying excessive system softness, a “spongy” actuating feel.
To that extent, in the case of the prior art devices for reducing pressure pulsations with the assistance of a profiled resilient membrane it is common to find a membrane contour leading to a degressive volume/pressure characteristic curve, i.e. in relation to low pressures (for example up to 5 bars) there is a relatively large volume absorption when the profiling is pressed against the fixed housing wall, whereas at higher pressures (for example 20 to 30 bars) and stronger pressing of the profile against the housing wall a significantly smaller volume can be taken up.
Against this background, it is clear that the prior art solution is capable of effectively reducing the afore-described “tingling” at lower pressures in the fluid column as a consequence of a relatively large volume absorption capability by the profiled membrane, but at increasing pressures in the fluid column progressively loses its damping effect by the profiled member due to a significantly diminishing volume absorption capability. However, it is also necessary to effectively reduce the transmission of pressure pulsations when the clutch pedal is depressed, i.e. at comparatively high pressures in the fluid column, for which reason further damping measures were provided in the prior art (additional line section with a helically extending helix section, cross-sectional narrowing with throttling effect, etc.), but these cause a degree of additional cost.
If it is additionally taken into account that there are vehicle-individual pressure regions, i.e. differing in dependence on the respective hydraulic actuating system and the vibration behavior thereof, with highest amplitudes of the pressure pulsation in the fluid column, a device for reducing pressure pulsations would be desirable which has a volume/pressure characteristic curve able to be steeper in vibration-critical pressure regions (higher volume absorption capability) than in the remaining pressure regions, where a flatter characteristic curve path (smaller volume absorption capability) would promise “stiffer” system behavior.
What is desired is a device for reducing vibrations in a hydraulic actuating system, particularly hydraulic clutch actuating system for motor vehicles, which has the simplest, most economic and most compact construction possible and is optimized with respect to its vibration damping characteristics produced by a profiled resilient membrane.